1. Field of the Invention
This invention relates generally to the field of digital communication systems and more particularly to deinterleavers employed in such systems.
2. Description of the Prior Art
An interleaver is commonly employed in a transmitter of a digital communication system for interleaving data such as bits or symbols. Correspondingly, a deinterleaver is commonly employed in a receiver of the digital communication system for reversing the interleaving process at the transmitter. The effect of employing an interleaver and a corresponding deinterleaver is that together they help to spread out local variations in the channel conditions more uniformly, so that the overall system performance is improved.
At the receiver, for instance, burst errors that occur due to noise or interference in the communication channel are spread out in time by the deinterleaver which can improve the performance of the forward error correction (FEC) decoder. Noise or interference may originate outside of the receiver, or may be caused by other blocks in the receiver itself. For example, a burst of errors may occur due to error propagation in a decision feedback equalizer (DFE). Interleavers also help in mitigating deep fades in a wireless communication channel. An interleaver is also an important component of turbo encoders where its use is a key contributor in the high performance of the codes.
Interleaving may be done at bit level or at symbol level. An example of a system that uses bit interleaving is the Orthogonal Frequency Division Multiple Access (OFDMA) mode of IEEE 802.16e standard. An example of a system that uses symbol interleaving is the Chinese Digital Terrestrial Multimedia Broadcast (DTMB) standard. See Chinese DTMB Standard, GB 20600-2006 (hereinafter referred to as the “Chinese DTMB Standard”).
Block and convolutional interleavers are two common types of interleavers. An example of a system that uses a block interleaver is the OFDMA mode of IEEE 802.16e standard for wireless communications devices. An example of a system that uses convolutional interleaver is the Chinese DTMB Standard More than one interleaver may be employed at the transmitter each of which may require a corresponding deinterleaver at the receiver. A single deinterleaver at the receiver may also be used to reverse the operations corresponding to multiple interleavers of interest at the transmitter.
The memory required for interleaving and the interleaver delay or latency of an interleaver are two important parameters of the interleaver. Often, the larger the delay in the interleaver, the better its capability in reducing errors. The term “total time span” of an interleaver or deinterleaver, as used herein, refers to the delay of the interleaver or deinterleaver in time dimension. However, the larger the interleaver delay, the more the memory required to interleave at the transmitter and to deinterleave at the receiver. In broadcast applications, delay or latency is not a major concern. Hence very large interleavers are used. Accordingly and undesirably, there is a large memory requirement for the deinterleaver in the receiver, which adds to the cost of the receiver.
Efficient quantization methods such as dynamic quantization and/or block floating point representation of signals are known to reduce the word size and thereby also the memory required. However these techniques do not change the number of quantities being stored themselves and hence reduction in memory that is achievable is limited. Accordingly, to reduce receiver cost significantly, there is a need to reduce the amount of memory used in the deinterleaver beyond such techniques.
When the interleaving operation of the interest at the transmitter is at the symbol level, there are two options for deinterleaving corresponding to this interleaving operation. Most communication systems transmit data in form of symbols to which bits are mapped. These symbols are received at the receiver with noise and other impairments.
In single carrier systems, the received symbols can be converted to bit soft metrics which can then be deinterleaved. Alternatively, the received symbols can be deinterleaved and then converted to bit soft metrics. In attempting to obtain reliable bit soft metrics from a symbol, symbol quality information (SQI) which is associated with that symbol is needed. The transmitter does not transmit SQI. Rather, SQI is information regarding a received symbol, such as the signal-to-noise and interference ratio (SINR) associated with the received symbol. The receiver may contain a block that determines this for each symbol. In addition to deinterleaving the received symbols, the SQI associated with the received symbols also has to be deinterleaved, and is then used to obtain the bit soft metrics. In methods currently used, SQI and data are interleaved and deinterleaved using similar structures, requiring storage of similar number of quantities or values. Accordingly, a large amount of memory is required for interleaving and deinterleaving, particularly, in systems that employ large interleavers at the transmitter, as discussed above. Using large amounts of memory is undesirable, because it increases the cost.
In bit interleaved systems, deinterleaving is performed on bit soft metrics that are generally obtained by making use of the SQI associated with them. Stated differently, the received symbols are converted into bit soft metrics by using the SQI associated with the respective received symbols. These bit soft metrics are deinterleaved. Both symbol and bit interleavers may be employed at a transmitter. In the absence of a symbol interleaver, current implementations do not separate the SQI prior to deinterleaving and the bit soft metrics that are deinterleaved already contain the effect of SQI in them. A large amount of memory is required for systems such as broadcast systems that use interleavers with large delays.
Therefore, the need arises for a method and apparatus to reduce the amount of memory used for deinterleaving at the receiver of a digital communications system thereby reducing cost without experiencing significant loss in performance.